The present invention relates generally to an exposure apparatus, and more particularly to a reflection type or catoptric projection optical system, and an exposure apparatus using the same which use ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light to expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
Recent demands for smaller and lower profile electronic devices have increasingly demanded finer semiconductor devices to be mounted onto these electronic devices. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed. It is expected to require circuit patterns of less than 80 nm in the near future. L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system for exposing a pattern on a mask or a reticle, onto a wafer. The following equation defines the resolution R of the projection exposure apparatus (i.e., a minimum size for a precise image transfer) where λ is a light-source wavelength and NA is a numerical aperture of the projection optical system:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the higher or finer the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light narrows usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflective elements, i.e., mirrors instead of refractive elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system cannot include any lenses. It has thus been proposed to form a catoptric projection optical system only with mirrors (e.g., multilayer mirrors).
A mirror in a catoptric reduction projection optical system forms a multilayer coating to enhance reflected light and increase reflectance, but the smaller number of mirrors is desirable to increase reflectance for the entire optical system. In addition, the projection optical system preferably uses the even number of mirrors to avoid mechanical interference between a mask and wafer by arranging them at opposite sides with respect to a pupil.
As the EUV exposure apparatus has requires a smaller critical dimension or resolution than a conventional one, higher NA is necessary (e.g., up to 0.2 for a wavelength of 13.4 nm). Nevertheless, conventional three or four mirrors have a difficulty in reducing wave front aberration. Accordingly, the increased number of mirrors, such as six, as well as use of an aspheric mirror, is needed so as to increase the degree of freedom in correcting the wave front aberration. Hereinafter, such an optical system is referred to as a six-mirror system in the instant application. The six-mirror system has been disclosed, for example, in Japanese Patent Applications, Publication Nos. 2000-100694 and 2000-235144.
Japanese Patent Application Publication No. 2000-100694 discloses some six-mirror catoptric projection optical systems with significantly large structures, because a fourth mirror M4 has a maximum effective diameter of 540 mm or greater relative to NA=0.2. Among the embodiments, the largest maximum effective diameter exceeds 650 mm relative to NA=0.28. The mirror's maximum effective diameter increases with NA. The mirror having the maximum effective diameter is arranged just before an intermediate image forming position, and the effective diameter increases because of large angles of view at the intermediate image forming position. In order for a ray from the fourth mirror M4 to a fifth mirror M5 not to interfere with a sixth mirror M6, these angles are inevitably large and becomes larger than sin−1NA.
On the other hand, Japanese Patent Application Publication No. 2000-235144 discloses a catoptric projection optical system that has small angles of view at the intermediate image forming position. In general, an effective diameter increases with a distance from a pupil, and a distant exit pupil from the intermediate image enlarges an effective diameter relative to small NA, such as NA=0.14.